A scanning electron microscope (SEM) is a common type of electron microscope. In an SEM, an energetic primary beam of electrons is focused on to and scanned in a raster-like pattern over a specimen. The primary beam causes electrons to be emitted from the specimen and induces current in the specimen. By detecting the emitted electrons or the induced current, a high magnification (typically higher than 100,000) image of the specimen can be produced on, for example, a CRT screen. A conventional SEM is constructed to detect emitted electrons, and not transmission electrons. Emitted electrons include backscattered electrons, secondary electrons, auger electrons, and induced current.
Other types of electron microscopes detect transmission electrons, those that travel through the specimen. For example, a scanning transmission electron microscope (STEM), scans a primary electron beam over a specimen, as in a conventional SEM. Unlike in an SEM, however, an STEM detects the transmitted electrons that pass through the specimen to produce an image of the specimen. A STEM usually operates with higher primary beam energies and is used for thinner specimens, in comparison to a SEM. The primary beam energy is typically in the range of 1 to 30 keV for SEM and 80 to 120 keV for STEM.
Because transmitted electrons react less with the specimen than non-transmitted electrons, a much higher image resolution can be obtained by detecting transmitted electrons. For example, at the top energy of 30 keV, the image resolution for a SEM is typically limited to 2 to 3 nm. In comparison, the resolution limit for a STEM can be as low as 0.2 to 0.3 nm.
Further, a STEM can be used with an electron energy loss spectrometer placed below the specimen. For example, an EELS can be a magnetic prism spectrometer having two parallel magnetic sector plates for creating a transverse magnetic field in the path of transmitted electrons. Transmitted electrons having different energies will bend to different angles under the same magnetic field. Therefore, a STEM with an electron energy loss spectrometer can selectively detect either those transmitted electrons that have not lost energy in the specimen (producing so-called brightfield images) or those that have lost energies due to interactions with the specimen (producing so called darkfield images). A STEM equipped with an electron energy loss spectrometer thus enhances its ability to provide quantitative material analysis.
Generally, it is more expensive to build and operate a STEM than a SEM. It is sometimes desirable to convert a SEM into an STEM for imaging thin objects (e.g. less than 50 nm thick). A known approach of converting a SEM to a STEM is to simply place a specially designed electron detector below the specimen. However, separate detectors need to be used to detect brightfield and darkfield images. Further, the image resolution using this approach is limited by the aberrations of the SEM objective lens.
Thus, there is a need for a simple way to convert a conventional SEM to a STEM that has high image resolution and/or can obtain energy loss spectral information.